The invention relates to printing apparatus and it is particularly concerned with printing apparatus such as an offset lithographic printing apparatus, which utilises a cylinder for applying print to a web.
Known offset lithographic printing machines comprise a plate cylinder to which ink is applied by rollers, a blanket cylinder which transfers the print from the plate cylinder to a web, and an impression cylinder which presses the web against the blanket cylinder. The blanket cylinder, will have a circumference corresponding either to the total print length or to a print length which is exactly divisible into the circumferential length of the blanket cylinder. Where a customer requires a print length which will not divide exactly into the circumferential length of the blanket cylinder, it is necessary to change the cylinder and, hitherto, it has been proposed to mount the blanket cylinder and plate cylinder in a cartridge which can be withdrawn axially to one side of the web. By doing that, the plate and blanket cylinders can be changed or cleaned easily.
If it is necessary to print on both sides of a web, it is usual to pass the web through a first print tower where print is applied to one side of the web, invert the web downstream of the first print tower and then pass the inverted web through a second print tower which applies print to the opposite side of the web. By doing that, the previously printed side of the web comes into engagement with the impression cylinder of the second print tower. As the printing is normally carried out at high speed, the ink applied to the first side of the web will often not be completely dry by the time it comes into engagement with the impression cylinder of the second print tower and ink will be transferred to that impression cylinder. Provided that the impression cylinder of the second print tower has a circumferential length which can be divided exactly by the print length applied by the first print tower, no problems arise. However, if the impression cylinder circumference is not so divisible, what is known as "ghosting" can occur where a second but lighter image of the print applied to the first side of the web is applied again to that side by the impression cylinder of the second print tower. Therefore, where the plate and blanket cylinders are changed in an upstream print tower it will generally be necessary to change both the plate and blanket cylinders and the impression cylinder of the downsteam tower in order to ensure that ghosting does not occur. In view of the fact that the web passes over the impression cylinder, it is necessary to remove the web from known machines when changing the impression cylinder which can be most inconvenient, particularly if numerous print towers are present in the machine. It is well known, that re-webbing a printing machine can be a long and complex operation and increases the time that the machine is out of service.
On certain prior art machines, it is known to provide the plate cylinder, blanket cylinder and impression cylinder as a single unit which can be mounted into or dismantled from a printing machine and an operator can carry various units having cylinders of set sizes. However, such units are very expensive as each one incorporates three cylinders and the unit is very heavy and difficult to install and remove. Also, it is essential to remove the web from the machine in order to replace the three-cylinder units.